1. Field of the Invention
This invention relates to a control system for a power conversion system, and more particularly to a control system for a power conversion system composed of voltage source type power converters which can continue its operation without generating overcurrents when the voltage waveform of an AC power source is distorted greatly due to a fault in the AC power source system.
2. Description of the Related Art
Hereinafter, in the drawings, like reference numerals designate identical or corresponding parts throughout the several views, but it is noticed that a part with a reference numeral not in parentheses is different from a part with the same reference numeral in parentheses.
FIG. 8 is a block diagram showing the construction of a conventional main circuit of a power conversion system. In FIG. 8, 1 is an AC power source, 2 is a first transformer, 3 is a second transformer, 4 is a first converter, 5 is a second converter, 6 is a capacitor, and 7 is a DC power source or a load. 8-19 are self-turn-off devices, such as, gate turn-off thyristors (hereinafter, referred to simply as GTO) composing first converter 4, 20-31 are diodes respectively connected in anti-parallel with GTOs 8-19, 32-43 are self-turn-off devices, such as, GTOs composing second converter 5, and 44-55 are diodes respectively connected in anti-parallel with GTOs 32-43.
FIG. 9 is a block diagram showing the construction of a conventional control system for the power conversion system shown in FIG. 8. In FIG. 9, 1 corresponds to AC power source 1 in FIG. 8, 4,5 is a voltage source type power converter corresponding to converters 4 and 5 in FIG. 8, and 2,3 is a transformer corresponding to transformers 2 and 3 in FIG. 8. Furthermore, 6 and 7 correspond respectively capacitor 6 and DC power source or load 7 in FIG. 8.
Further, 56 is a line-to-line voltage detector for detecting three line-to-line voltages of three-phase AC power source 1, and 65 is a phase locked loop (PLL) which generates a voltage phase value .theta. of AC power source 1, obtained by removing the distortion included in the voltage waveform of AC power source 1. 59 is a current control circuit for controlling output currents of converter 4,5. The output currents of converter 4,5 are detected by a current detector 68 and the detected three-phase currents are converted into two-phase currents by a three-phase to two-phase converter 69. The converted two-phase currents are inputted to a coordinate transformation circuit 70, which converts the two-phase currents into current values on a coordinate having the same phase as that of a voltage vector of AC power source 1 using voltage phase value .theta. of AC power source 1 computed by phase locked loop 65. Such current values are composed of an active current detected value 66 and a reactive current detected value 67 and are inputted to current control circuit 59. As current command values for current control circuit 59, an active current command value 72 and a reactive current command value 73 are inputted to current control circuit 59. 71 is a coordinate transformation circuit which converts output signals of current control circuit 59 into two-phase signals on a static coordinate system using voltage phase value .theta. computed by phase locked loop 65. The converted two-phase signals are inputted to a two-phase to polar coordinate transformation circuit 61 which computes an amplitude and a phase angle from the two-phase signals. The computed phase angle is inputted to a triangular wave generator 62 which generates triangular wave signals corresponding to the computed phase angle of 0.degree. to 360.degree.. The computed amplitude is inputted to a cross point detector 63 which detects cross points of the computed amplitude and the triangular wave signals and generates pulse width modulated signals for turning ON or OFF the GTOs in converter 4,5. The pulse width modulated signals are inputted to a gate pulse generator 64 which generates gate pulses for actually turning ON or OFF the GTOs in converter 4,5 based on the pulse width modulated signals. Here, two-phase to polar coordinate transformation circuit 61, triangular wave generator 62 and cross point detector 63 compose a pulse width modulation control means.
FIGS. 10 to 12 are waveform diagrams for explaining the operation of the conventional control system shown in FIG. 9. Hereinafter, the operation of the conventional control system shown in FIG. 9 is described with reference to FIGS. 10 to 12.
FIG. 10 shows waveforms of the voltage detected values of AC power source 1 through the outputs of two-phase to polar coordinate transformation circuit 61 and triangular wave generator 62. In FIG. 10, (1) is a UV-phase line-to-line voltage of AC power source 1, (2) is a VW-phase line-to-line voltage of AC power source 1, and (3) is a WU-phase line-to-line voltage of AC power source 1. It is assumed that at a time t1 two lines of the U-phase and V-phase are grounded and thereby a U-phase voltage and a V-phase voltage become zero. After time t1, UV-phase line-to-line voltage (1) becomes zero, and VW-phase line-to-line voltage (2) and WU-phase line-to-line (3) become the voltages whose amplitudes are 1/.sqroot.3 times those before time t1, respectively. (4) and (5) are two-phase AC voltage signals converted from three-phase line-to-line voltages of AC power source 1 and are used in phase locked loop 65 for detecting voltage phase angle .theta.. As phase locked loop 65 is provided with a filter having a considerably long time constant enough to eliminate the distortion included in the detected AC voltages, two-phase AC voltage signals (4) and (5) do not change practically their waveforms after time t1. (6) and (7) are output signals of current control circuit 59. (8) is an amplitude signal of two-phase to polar coordinate transformation circuit 61. (9) is one of triangular wave signals outputted from triangular wave generator 62 obtained by converting the phase angle signal from two-phase to polar coordinate transformation circuit 61 into triangular waves
(10) is a UV-phase voltage of converter 4,5 and (11) is a U-phase current of AC power source 1. These are explained later in detail with reference to FIG. 11
Next, with reference to FIG. 11, the operation of the control system shown in FIG. 9 is described from two-phase to polar coordinate transformation circuit 61 and triangular wave generator 62 through converter 4,5. In FIG. 11, in cross point detector 63, output signal (9) of triangular wave generator 62 is compared with amplitude output signal (8) of two-phase to polar coordinate transformation circuit 61. While the phase angle is in the range of 0.degree.-180.degree., when signal (8) is larger than signal (9), GTO 8 and GTO 15 in converter 4 are in the ON state and a positive DC voltage is applied to a secondary U-phase winding of transformer 2. When it is detected that signal (8) becomes smaller than signal (9), GTO 8 is turned OFF and GTO 14 is turned ON, and then the voltage applied to the secondary U-phase winding of transformer 2 becomes zero Next, while the phase angle is in the range of 180.degree.-360.degree., when signal (8) becomes larger than signal (9) GTO 15 is turned OFF and GTO 9 is turned ON, and thereby a negative DC voltage is applied to the secondary U-phase winding of transformer 2. When it is detected that signal (8) becomes smaller than signal (9), GTO 14 is turned OFF and GTO 8 is turned ON, and then the voltage applied to the secondary U-phase winding of transformer 2 becomes zero. Then, GTOs 8, 9, 14 and 15 are ON-OFF controlled the same as described above, the waveform of the voltage at the secondary U-phase winding of transformer 2 becomes as shown in (12). (13) is a U-phase output voltage of converter 5 and is applied to a secondary U-phase winding of transformer 3. U-phase output voltage (13) of converter 5 is obtained, similarly as described above, by turning OK and OFF GTOs 32, 33, 38 and 39 in converter 5 based on the comparison result of a triangular wave snot shown) lagged of the phase angle from triangular wave (9) by 30.degree. which is generated from triangular generator 62 with amplitude output signal (8).
Here, a primary winding of transformer 3 is of a zigzag connection. That is, a U-phase primary winding is composed of a series connection of a first winding positively coupled to a U-phase secondary winding by a turn ratio of 1/.sqroot.3 and a second winding negatively coupled to a V-phase secondary winding by a turn ratio of 1/.sqroot.3. (14) is a voltage induced in the first winding, that is, U-phase primary winding positively coupled to U-phase secondary winding in transformer 3, and the amplitude of voltage (14) is 1/.sqroot.3 times that of voltage (13). Similarly, (15) is a voltage induced in the second winding, that is, U-phase primary winding negatively coupled to V-phase secondary winding in transformer 3. Accordingly, a voltage induced in U-phase primary winding of transformer 3 is the sum of voltages (14) and (15). In U-phase primary winding of transformer 2, a voltage equal to voltage (12) is induced. Therefore, a sum of voltages induced in U-phase primary windings of transformers 2 and 3 is the sum of voltages (12), (14) and (15) and is shown by a voltage (16). Voltage (16) is a voltage obtained by composing the output voltages of converters 4 and 5 by transformers 2 and 3 and is referred to as a U-phase converter voltage. Similarly, (17) is a V-phase converter voltage. (10) is UV-phase converter voltage which is equal to the difference between U-phase converter voltage (16) and V-phase converter voltage (17). (18) is a UV-phase voltage of AC power source 1. U-phase current (11) flows which is determined by the relation of the difference between UV-phase converter voltage (10) and UV-phase power source voltage (18) (UV-phase line-to-line voltage (1) of AC power source 1) and the impedances of transformers 2 and 3.
FIG. 12 is a waveform diagram showing the relation among voltages of AC power source 1, voltages of converters 4 and 5, and currents flowing in AC power source 1.
In FIG. 12, (18) is UV-phase power source voltage (UV-phase line-to-line voltage (1) of AC power source 1), (20) is a VW-phase power source voltage (VW-phase line-to-line voltage (2) of AC power source 1), and (22) is a WU-phase power source voltage (WU-phase line-to-line voltage (3) of AC power source 1). (10) is UV-phase converter voltage, (19) is a VW-phase converter voltage, and (21) is a WU-phase converter voltage. Further, (11) is U-phase current, (23) is a V-phase current, and (24) is a W-phase current.
If at time t1, two lines of the U-phase and V-phase are grounded, thereby U-phase voltage and V-phase voltage become zero. After time t1, UV-phase power source voltage (18) becomes zero, and VW-phase power source voltage (20) and WU-phase power source voltage (22) become the voltages whose amplitudes are 1/.sqroot.3 times those before time t1, respectively. Output voltages (10), (19) and (21) of converters 4 and 5 can not follow the sudden change of these voltages and U-phase current (11), V-phase current (23) and W-phase current (24) become in the overcurrent state, respectively.
As described above, according to the conventional control system for a power conversion system, voltages of AC power source 1 are detected and are supplied to phase locked loop 65, which reduces the distortion included in the voltage waveforms of AC power source 1 and computes a voltage phase based on the AC voltages of less distortion. Converters 4 and 5 are controlled based on this voltage phase computed by phase locked loop 65. Therefore, when a fault, such as a line-to-ground fault, occurs in the AC power source system, it is impossible to detect the voltage phase thereof instantaneously by the affect of the filter with a large time constant included in phase locked loop 65. As a result, it is impossible to suppress the overcurrent caused by the sudden change of the voltages and thereby it is not able to continue the operation of the power conversion system.